 
 (defn convert-units-f [from to]
   (let [factors   {[:ft-lb :J]                1.35582
-                   [:m :ft]                   3.28084
                    [:kg :lb]                  2.20462262185
                    ;; slurry rate conversion not needed (use [:bbl :m3])
                    [:m3 :bbl]                 6.28981077}
         converter {[:kg-per-m3 :lb-per-cu-ft] #(/ % (factors [:lb-per-cu-ft :kg-per-m3]))
                    [:ft-lb :J]                #(* % (factors [:ft-lb :J]))
                    [:J :ft-lb]                #(/ % (factors [:ft-lb :J]))
-                   [:ft :m]                   #(/ % (factors [:m :ft]))
-                   [:m :ft]                   #(* % (factors [:m :ft]))
                    [:kg :lb]                  #(* % (factors [:kg :lb]))
                    [:lb :kg]                  #(/ % (factors [:kg :lb]))
                    [:bbl :m3]                 #(/ % (factors [:m3 :bbl]))

    [target-magnitude to-unit substance]))
(def energy-as (as-f :ft-lb :J 1.35582))
(def force-as (as-f :lbf :N 4.44822))
(def power-as (as-f :hp :W 745.69987158227022))
(def pressure-as (as-f :psi :kPa 6.894757293168361))
(def proppant-concentration-as (as-f :lb-per-gal :kg-per-m3 119.826))

  ([] (typical-stage-length (rand-length-unit)))
  ([length-unit]
   (condp = length-unit
     :ft (make-measurement (tuc/draw-normal 152 17) :ft)
     :m (length-as (typical-stage-length :ft) :m))))

(defn typical-stage-extent
  ([]
